Today is a momentous day for us, as Yahoo! has completed the process of moving search ad serving for Yahoo! to the Microsoft Advertising adCenter platform in the U.S. and Canada. Additionally, this marks the completion of the transition of advertisers’ Yahoo! Search Marketing accounts to adCenter.

From now on, to manage the ads displayed on Yahoo US or Canada, you need to manage them on Microsoft adCenter. Yahoo said that Yahoo! Search Marketing accounts will change to a read-only state, as early as tomorrow. And you will be able to access the Yahoo accounts over the next five months via the same log-in you have used previously, so that you may view historical data from the past 13 months.

Microsoft commented on the CPC prices, which we have seen spike up recently, due to the transition. Microsoft said, “As advertiser demand grows and drives competition in the adCenter auction, the increased competition may result in higher costs per click (CPCs) in the initial weeks following the transition. As advertisers experiment with bidding on each keyword to determine what works best for their campaigns, we expect these prices to stabilize within weeks.”

Here are some additional support notes from Yahoo:

Those advertisers who managed their Yahoo! Search Marketing accounts on a self-serve basis will now be exclusively supported by Microsoft. For account assistance, these advertisers should call (800) 518-5689, Monday through Friday between 4:00 a.m. and 9:00 p.m. Pacific Time, or Saturday between 6:00 a.m. and 6:00 p.m. Pacific Time.

If you are a YSM advertiser managing your account(s) through a Yahoo! account manager, your adCenter account support is now managed exclusively by your Yahoo! account team, which has been guiding you through the transition.

On the mobile search front, the mobile search ads will be transitioned to the Microsoft Platform by the end of October. However, internationally the transition will happen in 2011, with all global customers and partners expected to be transitioned by early 2012.